The present invention relates to a circuit board wherein devices having current levels substantially higher than other devices are isolated to reduce signal distortion.
In a computer, a backplane comprises a computer bus for transmitting signals between circuit boards coupled to the backplane by connectors and a backplane ground for diverting ground currents on the individual circuit boards to an external ground. The individual circuit boards typically comprise I/O devices, and logic devices configured to perform some function on the signals on the bus, e.g. a video board, a processor board, or an I/O board. The signals sent to the I/O devices from the bus have current levels substantially higher than the current levels associated with logic devices on the board.
In very high speed bus systems, these relatively high currents flowing between the I/O devices and the backplane through the connector may induce enough distortion in the signal currents on the board to render the board inoperable. The distortion may be caused by crosstalk between the ground currents of the I/O devices and the signal currents in the logic circuitry, and by noise generated in the signal traces connecting the I/O devices to the connector as a result of inductance in the signal traces or a characteristic impedance mismatch. Therefore, the I/O devices are physically and electrically isolated from the logic devices to reduce signal distortion caused by crosstalk and noise in the signal traces.
Generally, the I/O devices are connected through signal traces in such a manner that current loops formed by the signal terminal and ground terminal on the I/O devices in contact with the signal and ground pins on the connector have a short path length and a minimal amount of inductance. Additionally, the ground currents of the I/O devices are desirably returned directly to the ground pins on the connector whereas the ground terminals of the logic devices connect through the board to a ground plane on the bottom of the board which is in electrical contact with the ground pins of the connector.
In a typical circuit board 10, as shown in FIG. 1, a number of surface mounted devices including logic devices 12 having ground terminals 46 and signal terminals 48, and input/output (I/O) devices 14 having ground terminals 50 and signal terminals 52, are surface mounted on pads 16 disposed on insulating board 18. The logic devices receive signals from the I/O devices, process the signals, and send them back to the I/O devices, while the I/O devices interact with the logic devices, and send and receive signals from off the circuit board, e.g. from the computer bus. The current levels of the signals passed between the I/O devices and the computer bus are substantially higher than the current levels involving the logic devices. A signal plane 20 on the surface of insulating board 18 includes mounting pads 16 and signal traces 22 connected to the device terminal through the pads.
A signal plane 24 comprising signal traces 26 running parallel to each other lies below insulating board 18, separated by a layer of insulation 28 from a signal plane 30 also comprising signal traces 32 running parallel to each other but perpendicular to signal traces 26. An insulating board 34 is disposed below signal plane 30, the latter being provided with a copper ground plane 36 formed on its bottom surface.
Male connector 38 having ground pins 40 and signal pins 42 is mounted on the surface of insulating board 18, wherein ground pins 40 extend through the insulating boards and contact the ground plane 36, while signal pins 42 extend through the insulating boards and contact signal traces 26 and 32. A plurality of vias 44 are routed through the insulating boards connecting the signal traces 22, 26 and 32 to each other, and connecting the ground terminals of the logic devices to the ground plane. Signal traces 47 in signal plane 20 couple the ground terminals of the I/O devices to the ground pins 40 of connector 38.
The circuit board 10 is typically constructed by forming a layer of copper on the bottom of insulating board 34 to create ground plane 36, depositing a layer of metal on the top surface of insulating board 34, and etching the metal to define signal plane 30. Further layers of metal are deposited on the top and bottom surfaces of insulating board 18 and are etched to provide signal planes 20 and 24 respectively. Particular masks employed to etch signal planes 30, 24 and 20 determine the layout of the I/O and logic devices as well as the signal traces connecting the ground terminals of the I/O devices to the ground pins of connector 38. The two insulating boards 18 and 34 are joined by means of insulating layer 28 separating signal planes 24 and 30 with the vias 44 being formed by boring holes through the various insulating boards and filling the holes with metal.
A backplane 54, having a bus 57 (not shown in FIG. 1) and a backplane ground 55, receives circuit board 10 by coupling male connector 38 on the circuit board to a female connector 56 on the backplane. The connectors convey signals between the bus and the I/O devices on the board, and direct the ground current of the I/O and logic devices to the backplane ground in turn connected to external ground.
The I/O devices may comprise devices such as a turbotransceiver 58, designed for use in very high speed bus systems and shown schematically in FIG. 2. The turbotransceiver has signal pins 66 that communicate with the logic devices, a logic ground 68 connected to the ground plane 36 through one of the vias 44, I/O signal terminals 60 each being connected through one of the signal pins 42 to the bus 57, and a band gap ground terminal 62 as well as three bus ground terminals 64 connected through pins 40 to backplane ground 55. The currents between the connector 38 and terminals 60, 62, and 64 of the turbotransceiver are substantially higher in level than the currents in the logic devices and may induce enough distortion in the signal currents to render the circuit board inoperable. To reduce the distortion in the signals, the ground current from the I/O devices is physically and electrically isolated from the ground current in the logic devices, while the signal traces joining the ground terminals to the ground pins of the connector are configured to reduce the inductance in the current loop and the noise induced in the signals.
One method for isolating the I/O devices from the rest of the circuit employs a copper strip conductor formed around the edge of the top surface of insulating board 18, with the I/O devices spaced along the board adjacent the strip. The ground terminals of the devices are connected to the strip which is in contact with the ground pin of the connector, while the ground terminals of the logic devices are connected to ground plane 36 in contact with the ground pin of the connector.
FIG. 3 illustrates a known design for signal plane 20 of circuit board 10 connecting the ground terminals of the turbotransceivers to the ground pins of the connector. Turbotransceivers are grouped directly adjacent connector 38 such that the path length of a signal trace 47 connecting the ground terminal of the turbotransceiver to the nearest ground pin of connector 38 is as short as possible for reducing trace inductance. The ground pin is blackend out to show the connection between the ground pin and signal trace 47. Each of the ground terminals has its own signal trace 47 to minimize noise.
Referring to FIGS. 1 and 3, the aforementioned design creates a transmission line environment by locating signal traces 26 and 32 both above the ground plane 36. The signal traces and the ground plane form a transmission line that has a given characteristic impedance allowing a constant impedance to be established between the connector and the I/O devices for matching the impedance of the backplane and the connector. This constant impedance match is important in order to reduce noise in the signal traces as would be caused by an impedance mismatch. The value of the impedance is determined according to the distances between the ground plane and the signal traces, the width and height of the signal traces, and the dielectric constants of the insulating boards. It is kept constant by maintaining these factors.
Loop currents are developed by current flowing in through signal pins 42, through the signal traces, up the vias to the signal terminals of the turbotransceiver for example, through the transceiver circuitry, out the ground terminal through traces 47, and back to the ground pins on the connector. Currents flowing in these loops establish electro-magnetic fields that induce crosstalk in nearby signal leads thereby causing distortion. Returning the ground terminal of the I/O device to the ground pin nearest the associated signal pin reduces the path-length and the area of the current loop and the particular construction of the signal trace may reduce trace inductance.
However, the effectiveness of the above method for isolating I/O devices, such as the turbotransceiver, can be limited by the pin-to-pin connections of the ground terminals of the I/O devices to the ground pins of the connector, and by the transmission line environment. For high speed bus signals, the separate signal traces may not provide suitable electrical properties such as resistance, capacitance and specifically minimum inductance for effectively connecting the I/O devices to the connector without causing distortion. Also, as shown in FIG. 3, when several I/O devices are grouped adjacent the connector, routing the traces between the terminals and the ground pins for maintaining a low inductance path may be difficult. Furthermore, the transmission line environment created on the board in the area of the high current I/O devices tends to reinforce the electro-magnetic fields established by the high current loops that cause crosstalk and signal distortion.